Conventionally, files and directories in a storage subsystem are backed up with file-based operations, which build individual files and directories on tapes by going through hierarchical file system structures. These conventional backup techniques generally have limited backup performance due to various file system overheads. For example, a backup operation for small files, dense directories, or fragmented file locations generally involves small reads and random disk access, which in turn incur a significant file system overhead.
Further, file-based backup techniques need to back up a file on a file-by-file basis. The files often have to be backed up in certain order, such as inode-based ordering and directory tree based ordering. For each file, file-based backup techniques have to backup the data from the beginning to the end. The constraint imposed by the ordering limit the performance. For example, the dump format of Berkeley Software Distribution (BSD), further imposes strict ordering constraints among files, as well as data blocks of a file.
Additionally, file-based backup techniques are often unable to provide a sufficient data input rate to a tape drive, which causes a shoe-shining effect to occur. The shoe-shining effect occurs during tape reads or writes, when the data transfer rate falls below a minimum threshold at which the tape drive heads are designed to transfer data to a running tape. When the shoe-shining effect occurs, the tape drive stops, rewinds back the tape, accelerates again to a proper speed, and continues writing from the same position. The shoe-shining effect significantly reduces the backup performance.
There are additional problems with file-based backup. For example, file-based backups do not preserve metadata used by the storage system. Although a restore operation will restore user data, it cannot restore the metadata in the original volume. Loss of the metadata may result in loss of the functionality that users may have on the original volume.